1. Field of the Invention
The present invention relates to an industrial robot and more particularly to an industrial robot having at least three articulated pivotal joints. The present invention can most suitably be applied to a welding robot for carrying out spot welding in a motor vehicle body assembling line but the invention is not limited to such an application.
2. Description of the Prior Art
In the motor vehicle industry, there is an increasing trend of manufacturing the vehicle body by welding thin steel sheets for decreasing the body weight and providing the body with a required rigidity. Spot welding is commonly adopted for the purpose. Recent tendency is that the spot welding is made with finer spacings than used to be so that number of spot welding is substantially increased. As an example, approximately 5000 spot weldings are made in each one of vehicle bodies. It is therefore unavoidably required in a vehicle body assembling line to provide a spot welding line of a substantial distance. From the viewpoint of effective utilization of the factory space, however, it is advisable to make the spot welding line as short as possible.
A typical example of spot welding robots adopted in vehicle body manufacturing lines is shown in FIGS. 10 and 11. As shown, the robot includes a stationary base 1 supporting a swivel body 2 for swivel movements about a vertical axis 3. On the swivel body 2, there is mounted a vertical swing arm 5 for fore-and-aft swinging movements about a horizontal axis 6. On the upper end portion of the arm 5, there is mounted a horizontal swing arm 8 for vertical swing movements about a horizontal axis 7. The horizontal arm 8 carries a hand 9 having welding electrodes at a tip end. A link 11 is provided to extend in parallel with the arm 5 and connected at an upper end with a rearward end portion of the horizontal arm 8. The link 11 has a lower end which is connected with a horizontal link 11a which extends rearwardly from the axis 6 in parallel with the arm 8.
As shown in FIG. 11, the swivel body 2 is provided with a driving motor 4 for driving the arm 5 about the axis 6. The arm 5 is swingable in the fore-and-aft direction through an angular range 82. The swivel body 2 is further provided with a driving motor 10 which is arranged to drive the link 11a so that the link 11 is moved in a vertical direction to thereby make the horizontal arm 8 swing in the vertical direction through an angular range .theta.3. Appropriate operations of the motors 4 and 10 cause swinging movements of the arms 5 and 8 so that the robot hand 9 can be moved in the region designated by the reference numeral 13.
This type of welding robot is considered as being advantageous in that the robot hand can be moved in a wide range with a relatively small robot body. It should however be noted that the welding robot of this type still has a disadvantage which will be described below. In FIG. 10, the rearward end portion of the arm 8 is shown by the reference numeral 12 when the arm 8 is in the horizontal position and by the reference numeral 14 when the arm 8 is in the fully upwardly swung position. It will therefore be understood that the rearward end portion 14 of the arm 8 projects by a distance R1 from the vertical axis 3 around which the swivel body 2 is rotated.
It will be noted that in this structure the rear end portion 14 of the arm 8 moves along a circular path of a radius R1 with a center at the axis 3 when the robot is rotated about the axis 3. In a typical vehicle body welding line, a plurality of robots 16 are arranged side by side with relatively small spacings between each adjacent two robots as shown in FIG. 12 at each side of a conveyor along which vehicle bodies 15 are moved. The aforementioned movement of the rear end portion 14 of the arm 8 may then cause an interference with the adjacent robot so that a larger spacing between each two adjacent robots must be provided for avoiding such possible interference.